In a conventional photodetector, such as, for example, a back-biased P-N diode, a photon of incident radiation dissociates an electron-hole pair. Each part then migrates to its majority region, producing a momentary current in an external circuit. The current, thus detected, must then compete for recognition with the noise current of the following amplifier or mixer stage.
A measure of a detector's ability to distinguish between the detected signal and noise is given by the detector's signal-to-noise ratio (S/N) which, in a large measure, serves to determine the basic operating parameters of an optical communication system. For example, for a given error rate, the signal-to-noise ratio is a factor determining the spacing between transmitter and receiver for a given transmitted power. The higher the ratio, the greater the spacing and/or the lower the permissible transmission power.
Aside from improvements in the design of the photodetector itself, various circuit techniques have been devised to improve the S/N ratio of the detector. One such technique, as typified by U.S. Pat. Nos. 3,196,274 and 3,379,888, is to lower the frequency of the detector output signal by means of a resistive mixing process which occurs when a local oscillator signal is injected into the detector circuit. A difficulty with this approach is that while it places the detected signal in a lower frequency range, which may have some advantages vis-a-vis the detector itself, it also places the detected signal in a noisier portion of the following amplifier's operating region. Thus, the advantages of this approach are clearly limited.
An alternate technique is to combine photodetection and parametric amplification in the same device, as described in pages 1017-1018 of the book entitled Coherent Light by A. F. Harvey, published by Wiley-Interscience, 1970. However, as reported therein, no significant improvements appear to have been realized as a result of this particular application of parametric interaction.